1. Field of the Invention
This invention relates to a method for separating the luminance and chrominance signals of a PAL or SECAM composite video signal and to a device for the practical application of said method. The invention is applicable to television receivers operating on PAL or SECAM standards as well as to any television equipment in which the luminance and chrominance signals are separated by means of a composite video signal.
2. Description of the Prior Art
In all color television systems, provision is made for a separation circuit which makes use of the composite video signal in order to deliver the luminance signal which represents the light intensity at each image point as well as the chrominance signal which represents the color of a point or group of points.
In the SECAM process, this separation is carried out by means of analog filters. The filter which restores the chrominance subcarrier is called a "bell" filter with reference to the shape of its curve of gain as a function of frequency. The filter which delivers the luminance signal is designated as a chrominance subcarrier rejector since it eliminates said subcarrier, the frequency of which is usually 4.286 MHz. This method of separation is attended by a certain number of drawbacks. In the first place, the separation is imperfect and gives rise to chromatic crosstalk phenomena known as "cross-color" and "cross-luminance". In the second place, restoration of the luminance information which is present in the composite video signal is lost at the frequencies which are eliminated by the chrominance subcarrier rejection filter. Finally, like any device of the analog type, this device is subject to problems of adjustment and instability.
In the case of the SECAM system, a circuit has been proposed for separating luminance and chrominance signals in which the conventional analog method outlined in the foregoing is carried out by digital processing means. A circuit of this type is described in French patent application No. 81.08343 filed in the name of Thomson-Brandt and the improvements obtained are limited to those achieved by digital techniques with respect to analog techniques, namely absence of adjustment, stability, insensitivity to noise.
The conventional method employed for separating luminance and chrominance signals in the PAL system is identical with the method employed in the SECAM system. By means of the analog composite video signal, the luminance signal is extracted by attenuating the chrominance signal located in the upper portion of the spectrum of the composite signal whilst the chrominance signal is obtained by means of a bandpass filter for selecting the upper portion of the spectrum of the composite signal. These two filtering operations are carried out by means of analog techniques and the disadvantages observed are those already mentioned in the case of the SECAM system.
In the case of the PAL system, a certain number of alternative techniques have been proposed for separating luminance and chrominance signals. The common principle of these techniques consists in filtering the composite video signal by means of a vertical comb filter of the type described, for example, in European patent Application No. 83.107.430.7 and No. 83.107.431.5 filed in the name of Tokyo Shibaura Denki Kabushiki Kaisha. These techniques turn to useful account the fact that, in the PAL system, the chrominance signals are in phase opposition when a two-line shift is produced in a given frame. It is possible by means of the same techniques to eliminate the cross-color and cross-luminance phenomena mentioned earlier and to restore the high-frequency luminance information in image areas which do not contain vertical transitions. A disadvantage, however, lies in the fact that false colors and transition shifts appear in the vicinity of vertical transitions.